Glass Shell Manufacturing in Space
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GLASS SHELL MANUFACTURING IN SPACE ROBERT L. NOLEN, RAYMOND L. DOWNS AND MATTHIAS A. EBNER KMS Fusion, Inc., P.O. Box 1567, Ann Arbor, Michigan, USA ABSTRACT Highly-uniform, hollow glass spheres, which are used for inertial-confinement fusion targets, are formed from metalorganic gel powder feedstock in a drop-tower furnace. The modelling of this gel-to-sphere transformation has consisted of three phases: gel thermochemistry, furnace-to-gel heat transfer, and gravity-driven degradation of the concentricity of the molten shell. The heat transfer from the furnace to the free-falling gel particle was modelled with forced convection. The gel mass, dimensions, and specific heat as well as furnace temperature profile and furnace gas conductivity, were controlled variables. This model has been experimentally verified. In the third phase, a mathematical model was developed to describe the gravity-driven degradation of concentricity in molten glass shells. INTRODUCTION Targets currently used in inertial confinement fusion (ICF) include spherical glass shells filled with deuterium-tritium fuel. Currently these shells have diameters from 100 to 500 ým, and wall thickness from 0.5 to 10 pm. To meet the experimental requirements the non-concentricity of the inner and outer surfaces of the shells must be less than 5%, asphericity less than 1% and surface irregularities less than 0.5 pm. These specifications are becoming more stringent as the available laser power increases and laser fusion experiments become more sophisticated. In addition, the shells are required to have high strength and chemical 4durability and a range of permeability for various diagnostic and fuel gases. Meeting these requirements becomes particularly difficult for the next generation of targets whose diameters are greater than lmn, the effective size limit for current manufacturing methods. For such large, massive glass shells, the force of gravity and the aerodynamic forces, acting upon falling molten shells, will increasingly manifest themselves, degrading the concentricity and sphericity of the shells, and by virtue of the terminal velocity of the gel, limit the glass fining time interval during which a falling particle is exposed to the heat of the furnace. These shells are currently manufactured by several methods [1,2,3]. One of the more promising and versatile methods involves the use of metal-organic powder which is fed into an electrically-heated vertical tube furnace whereby glass shells are formed. Metal-organic gels as glass precursors permit the formation of hollow glass spheres with a variety of glass compositions [4,5,6]. In view of the limitations on shell size and quality that are imposed by the gravitational and related aerodynamic forces on the current methods, the manufacture of shells in the near-weightless environment of space seems to be a viable alternative to earth-based manufacture of large-diameter highly-uniform shells. A necessary preliminary to any glass manufacturing experiments in space is a better understanding of the
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